The invention relates to optical devices and more particularly to a microlens structure or array over a photosensitive array.
Description of Related Art
Digital imaging systems, such as, for example, digital cameras, utilize integrated circuit devices or chips as image capturing devices. An imaging system, such as a camera, uses light to capture an image on a semiconductor-based chip. The chip replaces film in traditional film-based systems. In a digital camera, an image sensor is configured, in its simplest form, to capture a monochrome or color image by way of field effect transistors (FETs), such as complementary metal oxide semiconductor (CMOS) devices or charge coupled devices (CCDs).
In one example, the image sensor comprises a semiconductor chip made up of a number of photosensitive circuits, each photosensitive circuit capable of absorbing light. In color applications, each photosensitive circuit generally absorbs light through a color filter that represents a particular wavelength of light in the visible spectrum corresponding to the image sensed.
The photosensitive circuits of an image sensor, often referred to as pixel circuits, are generally arranged in an array, such as for example 480 rows by 640 columns. In general, each photosensitive circuit contains a photosensing element, such as a photodiode or charge-coupled device (CCD), and other circuitry. The photosensing element defines a photosensing region or area of the photosensitive circuit that responds to light while the circuitry, generally speaking, drives a light signal from the photosensing region or area to other process circuitry.
One method of converting a monochromatic digital imaging system into a color imaging system involves absorbing light through a color filter. The color performance of any color filter concerns the ability of the filter to select color corresponding to the desired wavelength of the visible spectrum of the color filter array (CFA) material. A common color filter material is spin coated-, dyed-, or pigmented-photoresist CFA material.
In order to improve the light collecting efficiency of a photosensing circuit, a microlens is typically formed on top of the CFA material overlying each photosensitive circuit. A planarization layer having high transparency properties may also be deposited between the color filter array and the microlens material.
The microlens material is typically a photoresist. Initially, even the generally transparent photoresist is initially yellow or otherwise not entirely transparent after formation. The lack of transparency is generally attributed to the photosensitivity component (such as a photoacid compound) of the photoresist material. In order to increase the transparency, the photoresist is often bleached. Photobleaching occurs after the deposition and patterning of the microlens material, for example, as a photobleaching with ultraviolet light to cross-link the photoresist molecules and destroy the photosensitivity of the microlens. In general, the greater the cross-linking and the destruction of the photosensitivity of the microlens, the greater the transparency clarity of the microlens. An incomplete photobleaching will result in a microlens that is not completely transparent or that is yellow.
Another problem with using photoresist material as the microlens material is that temperatures greater than 150.degree. C. tend to degrade the photoresist material and cause the deformation of the microlens shape. Many steps in forming an image sensor, however, sometimes include heating to greater than 200.degree. C., such as surface mount processes to couple the sensor package to a printed circuit board (PCB). Such temperatures therefore may damage or destroy the microlenses and the benefits desired with the incorporation of the microlenses. Thus, the instability at high temperatures of photoresist-based microlens material has generally restricted image sensor packaging on printed circuit boards to manual processes such as local heating solder processes in which package leads are connected through soldering while the sensor with CFA material and microlens material is kept at a lower temperature.
What is needed is an improved microlens.